Elevator systems include an elevator car that moves between various landings to provide elevator service to different levels within a building, for example. A machine includes a motor and brake for selectively moving the elevator car to a desired position and then maintaining the car in that position. A machine controller controls operation of the machine to respond to passenger requests for elevator service and to maintain the elevator car at a selected landing in a known manner.
One challenge associated with elevator systems is maintaining the car at an appropriate height relative to a landing to facilitate easy passage between the elevator car and a lobby where the elevator car is parked. The car floor is ideally kept level with the landing floor to make it easy for passengers to move between the lobby and the elevator car while minimizing the possibility of someone tripping. Current elevator codes define a displacement threshold that establishes a maximum difference that is allowable between the landing floor and the elevator car floor. When that distance is above the code threshold, the elevator system must re-level or correct the position of the elevator car.
The conventional elevator re-leveling approach includes sensing the amount of car-to-floor displacement. This is typically accomplished using an encoder on the primary position transducer or governor associated with the elevator car. When the displacement exceeds a set threshold, a re-leveling process begins. The machine controller makes a determination regarding the weight of the car and pre-torques the motor for lifting the car before releasing the machine brake. The motor current is then controlled using an inner velocity servo loop that has an outer position loop using a fixed gain feedback compensator (such as a Proportional plus Integral control) on the position error.
The conventional approach to re-leveling an elevator car works well in most situations. In high rise buildings and ultra high rise buildings, the conventional approach may not provide satisfactory results. This occurs, in part, because the effective stiffness of elevator roping members decreases proportionally with their length. Accordingly, a longer elevator roping arrangement allows for increased amounts of static deflection responsive to changing loads on the elevator car, which results from passengers entering or exiting the car, for example. Additionally, there is time delay between motor action, car reaction and position transducer response that increases linearly with the height of the hoistway. Such a delay introduces potential stability issues in the position feedback logic associated with the conventional approach. Another issue is that the reduced stiffness of the roping arrangement reduces the resonant frequency associated with elevator car bounce resulting from changes in the load on the car. The lower frequency resonance creates a limitation on traditional control logic gains, which limits bandwidth and, therefore, performance.